1. Field of the Invention
The present invention relates to an ultrasonic diagnostic equipment and an image processing apparatus, and more particularly to an ultrasonic diagnostic equipment and an image processing apparatus which offer information reflective of the blood flow velocity of a myocardial tissue by employing contrast echoes.
2. Description of the Prior Art
In the field of ultrasonic diagnostic equipments, the spotlight of attention has recently been focused upon contrast echocardiography wherein, in examining the heart, an abdominal organ or the like, an ultrasonic contrast medium is injected from a vein so as to estimate a blood flow movement. The technique of injecting the contrast medium from the vein is less invasive than the technique of injecting the same from an artery, and diagnoses based on the estimation method are coming into wide use. The ultrasonic contrast medium is principally constituted by microbubbles, which form reflection sources reflecting ultrasonic signals. As the dose or density of the contrast medium is higher, a contrasting effect enlarges more. In relation to the property of the bubbles of the contrast medium, however, such a situation occurs that the duration of the contrasting effect is shortened by ultrasound projection.
In case of performing the contrast echocardiography, the contrast medium is successively supplied to the region of interest (ROI) of a patient part by a blood flow. It is therefore supposed that, even when the bubbles have been once caused to disappear by projecting an ultrasound, the contrasting effect will be maintained if new bubbles flow into the ROI at the time point of the next ultrasound projection. In actuality, however, ultrasounds are usually transmitted and received several thousand times in one second, and an organic parenchyma of low blood flow velocity or the blood flow movement of a comparatively fine blood vessel is existent. In consideration of these facts, the bubbles will disappear in succession before intensity enhancement based on the contrast medium is confirmed on the image of such a diagnosis, with the result that the contrasting effect will weaken instantly.
The most fundamental one of diagnostic method among those employing the contrast medium consists in that the amount or presence of the blood flow of the part to-be-diagnosed is found by checking the presence of the intensity enhancement based on the contrast medium.
Further, an imaging technique which is called “flash echo imaging method” (or also called “transient response imaging method”) has been proposed by utilizing the phenomenon that the microbubbles disappear by the projection of the ultrasounds as stated above, and it has been reported that the intensity enhancement can be bettered by the imaging method (refer to, for example, Patent Document 1). In principle, the imaging method is a technique wherein the conventional continuous scan of, e.g., several tens frames in one second is replaced with the intermittent transmission of, e.g., one frame in several seconds. The microbubbles densified without being burst during the time interval of the intermittence are extinguished at one time, thereby intending to obtain a high echo signal.
In myocardial contrast echocardiography, however, enhancement is nonuniform due to nonuniformity in the acoustic field of a sector probe, and nonuniformity in an acoustic field as is induced by an in-vivo structure. The nonuniformity forms an obstacle to the diagnosis of myocardial ischemia or a myocardial infarction part.
Here, the “nonuniformity in the acoustic field of the sector probe” is as stated below. In the sector probe which is generally used in the examination of a circulatory organ employing an ultrasonic diagnostic equipment, transmission conditions are constant for individual scan lines constituting an image. Accordingly, when the deflection angle of a beam enlarges, there arise the phenomena in which a transmission acoustic field differs depending upon the scan lines, 1) that the acoustic pressure of the transmission beam formed on the scan line lowers, and 2) that a beam width becomes thicker than in a case where the transmission beam is not deflected, so a spatial resolution degrades.
In the prior-art B-mode scan of an ultrasonic beam, therefore, the endeavor of generating a more uniform image is made as a measure against the phenomena, by adopting such a technique as adjusting factor of the images such as a reception gain for every scan line on the basis of a reception signal from the nonuniform transmission acoustic field.
However, the above countermeasure is directed toward the case of the B-mode image, and it does not suppose the case of tissue harmonic imaging (THI) having come into the limelight in recent years, or harmonic imaging such as contrast echo imaging, so that drawbacks as stated below are involved.
In the harmonic imaging, the transmission acoustic field, especially the acoustic pressure, becomes an important factor for a sensitivity, and it has its limit to correct the nonuniformity of the transmission acoustic field by the image adjustments of the reception. Therefore, the uniformity of the transmission acoustic field is required for satisfactorily demonstrate the effects of the adjustments. The reason therefor is that, since the intensities of harmonic components are proportional to the square of the acoustic pressure, the reception gain the amplitude of which is the square of that in the prior-art B-mode must be corrected, so the change of a noise level is induced by the change of the reception gain, whereby a nonuniform image in which the noise level differs is generated.
Besides, irrespective of the harmonic imaging, the beam width generally thickens when the deflection angle of the transmission beam enlarges. This phenomenon induces the nonuniformity of the spatial resolution. Especially in a case of parallel simultaneous receptions, the phenomenon of beam bends might be increased.
On the other hand, the “nonuniformity of the acoustic field attributed to the in-vivo structure” is induced in such a manner, for example, that a rib exists in close proximity to the probe, so the transmission and reception of ultrasounds are hampered by the rib. In particular, since part of the left ventricle is located so as to be covered with the left lung, the lung intervenes between the probe and the myocardium, to often bring about the result that the corresponding part darkens on an image obtained by an ultrasonic diagnosis (refer to FIG. 1A).
Among the nonuniformities, the nonuniformity of the acoustic field of the sector probe can be theoretically corrected, but the nonuniformity of the acoustic field which is induced by the in-vivo structure cannot be corrected for the reason that the structure differs depending upon patients, so a theoretical solution does not exist. Therefore, it becomes difficult to detect a morbid part, such as myocardial ischemia or myocardial infarction part, which is clinically to be found.
Meanwhile, a technique for diagnosing the ischemic malady of the heart is called “stress echocardiography”. The technique is such that a vasodilator drug, for example, ATP (adenosine triphosphate) is injected into the myocardium by an intravenous drip or the like so as to expand blood vessels, whereupon a contrast medium is injected instead of the vasodilator drug, thereby intending to obtain an ultrasonic image.
According to the technique, a normal part becomes brighter than before because the blood vessels expand thereat. On the other hand, a morbid part undergoes the steal phenomenon of darkening contrariwise, for the reason that, since the blood vessels are difficult to expand at the morbid part, a blood flow increases to surrounding blood vessels which have expanded.
In a case where the blood vessels are not completely closed and where they become fine, a chest pain is sometimes incurred by a violent motion. In this regard, the technique is intended to observe such a part appearing as a defect (refer to FIG. 1B).
Even with this method, however, parts I′ and II′ sometimes seem to be ischemic regions as shown in FIG. 1B by way of example. That is, even with the stress echocardiography, it is very difficult to judge an ischemic region from a single image.
In order to solve such problems, a method is considered in which ultrasonic images of two or more frames are taken so as to be compared and studied. For this purpose, after the first image has been first obtained by injecting the contrast medium, the second image is taken by injecting the vasodilator drug and expanding the blood vessels, whereupon both the images are compared. Even with this method, however, the visual inspection of a diagnostician has its limit, and if a certain part is morbid is disputable and suspicious. Therefore, a method for comparing and arithmetically operating the two images is required. It is an actual situation to fulfill the requirement by executing the arithmetic operation between the different images.
Such an arithmetic operation is digital subtraction in which the images of the two frames are subjected to differential processing (refer to, for example, Japanese Patent No. 3,023,290). This method, however, is employed in a case where the temporal difference between the two B-mode images is usually correspondent to several frames, that is, about 1/20- 1/10 second in terms of time, so that it is hardly influenced by the pulsatory motion of an internal organ, the motion of a probe, or the like. Accordingly, only echoes caused by the contrast medium can be displayed on the differential image between the images of the two frames.
In contrast, especially in the stress echocardiography which compares the images before and after the stressing, a time interval is greater than in the mere flash echo imaging whose time interval is as short as several cardiac beats, and the positional shifts of the heart attributed to the motion of a patient himself/herself, how to apply the probe, etc. are not negligible. When the arithmetic operation is directly executed without making positional corrections, such a situation takes place where a motion artifact appears or where quite no operated result is obtained.
Further, ROIs (regions of interest) need to be separately set for the two images. In this regard, in a case where both the set ROIs are inaccurate, that is, where they do not satisfactorily correspond to each other, an accurate operated result cannot be expected.